Field of the Invention
The present invention relates to a fixing apparatus used in an electrophotographic image forming apparatus, such as a copier and a printer.
Description of the Related Art
An external heating fixing apparatus is proposed as a fixing apparatus used in an electrophotographic copier or printer. An external heating fixing apparatus usually includes a roller, a heating unit for heating the roller from outside, and a backup unit which forms a nip portion with the roller (see Japanese Patent Laid-Open No. 2004-258536).
The fixing apparatus is provided with a safeguard against loss of control of the heater due to a failure of a power control circuit, such as a CPU runaway or a short-circuit of a triac which controls the power supplied to the heater provided in the heating unit. The safeguard may be a power shutdown member, such as a thermoswitch and a thermal fuse, and a means for detecting an abnormal temperature by a thermistor and the like and shutting down the power supply to the heater. In the related art, the thermoswitch, the thermal fuse, the thermistor, and the like are provided near the heater which is the heat source (i.e., on the heating unit side) to facilitate sensitive reaction to an abnormal temperature rise of the apparatus.
In the external heating fixing apparatus, however, a backup unit is provided distant from the heating unit. Therefore, if an apparatus runaway occurs and an abnormal temperature rise is caused, it can be difficult to keep the temperature of the backup unit not to exceed a heatproof temperature using a safeguard provided in the heating unit.